1. Field of Invention
The present invention relates to liquid crystal displays and, more particularly, to a backlight assembly having an inverter for powering a plurality of discharge tubes, such as cold cathode fluorescent lamps, as a light source for the display.
2. Description of the Related Art
Conventional liquid crystal displays (LCDs) are made to be light-weight, compact and have low power consumption. Since the LCD is a non-emissive device, a light source such as cold cathode fluorescent lamp is used. The cold cathode fluorescent lamp is a kind of a fluorescent lamp that operates in the regular glow discharge region using an applied AC voltage. Since the cold cathode fluorescent lamp is not preheated by a filament, it is relatively more vibration resistant, has a thinner diameter and a longer life span compared to a hot cathode fluorescent lamp but requires a higher voltage to be applied, for which an inverter circuit is used.
As shown in FIG. 18, the conventional inverter circuit includes first and second inverters 11 and 12, a plurality of balance transformers 13, and a plurality of diodes 14.
Primary coils 13a of the balance transformers 13 are connected to output terminals of the first and second inverters 11 and 12. The first and second inverters 11 and 12 convert DC voltage into AC voltage and supply the AC voltage to two pairs of cold cathode fluorescent lamps 20 through the primary coils 13a of the balance transformers 13.
Among the secondary coils 13b of the balance transformers 13, adjacent secondary coils 13b are serially connected to each other to form a secondary coil serial loop, one end of the secondary coil serial loop being connected to the ground GND.
First ends of third coils 13c of the balance transformers 13 are connected to the ground GND through the secondary coil serial loop and second ends of the third coils 13c of the balance transformers 13 are connected to input terminals of the diodes 14.
Input terminals of the diodes 14 are connected to the first ends of the third coils 13c. The diodes 14 detect voltage generated from the third coils 13c and then generate voltage detection values.
Japanese Patent Unexamined Publication No. 2005-267923 discloses an error detection circuit employed in a discharge tube control circuit that controls the turn on/off of the first and second discharge tubes. An error signal generating device generates an error signal when the current balance of the first and second discharge tubes is different from the reference current balance.
However, as shown in FIG. 19, since the cold cathode fluorescent lamps are aligned between a diffusion plate and a reflective plate, when the cold cathode fluorescent lamps are turned on, heat is transferred upward by convection along the cold cathode fluorescent lamps causing the internal temperature of the backlight assembly to rise and lowering the lamps' impedance. FIG. 20 shows that, as the cold cathode fluorescent lamps generate heat, an impedance gradient occurs in the backlight assembly caused by the temperature gradient.
The conventional inverter circuit includes a circuit that detects voltage at the diodes connected to the third coils when the impedance changes due to the temperature gradient. The voltage value due to the impedance change must be taken into consideration when setting the threshold value used to detect the normal operation and abnormal operation. As a result, the conventional inverter circuit has a higher threshold value because of the increase of the voltage value caused by the impedance change according to the temperature gradient. For this reason, when the voltage is slightly increased due to arc discharge caused by an opening fault, the voltage level may not reach the threshold value, so that the voltage variation with temperature cannot be precisely detected.